Selective hydrogenation of a mixture of c5 hydrocarbons



Sept. 22, 1964 H. Hummmn smc'rrvl: Hmocm'rron or A mmm: or cmnocmons F1106 Hav. 12, 195i INVNTOR. HE|N2- VHENEMANN (fw -mnnt AGENT SELECTli/lil HWDRUGENATHN F A MIX'I'URE F tC HYDRUCARMUN@ lHeinz Heinemann, lUpper Montclair, NJ., assignor, by

mesne assignments, to Pullman Incorporated, a corporation of Delaware Filed Nov. 112, 1959, Ser. No. 052,353 t Claims. (Cl. Imi- 677) This invention relates to the separation and/or the production of useful products from low-boiling hydrocarbon mixtures and particularly mixtures of C5 hydrocarbons.

In a specific aspect this invention is directed to the selective hydrogenation of a mixture of C5 hydrocarbons to recover a product stream rich in trimethylethylene which may be readily separated and converted to isoprene.

The commercial preparation of isoprene (2 methyl 1,3 butadiene) has not been widely practiced in View of the diiculties in obtaining a suitable feed material and/0r the separation of isoprene from other C5 oleiins and dioletins. Since the C5 cut of cracked gasoline contains appreciable quantities of close boiling range oletins and parafns, the separation of the C5 hydrocarbon cut is particularly ditlicult because of the proximity of boiling points of 2-methylbutene 2 and pentene-2 (cis and trans). In addition, the volatilities of the corresponding azeotropes do not permit a feed separation by distillation. Thus the problem of recovering a suitable trimethylethylene-rich feed for conversion toisoprene from a mixture of C5 hydrocarbons consists in the recovery of the branched chain olefin from mixtures containing 2-pentenes.

Accordingly, the present invention is directed, in one embodiment, to a method of separating a mixture of straight and branched chain C5 olefins by selectively hydrogenating the straight chain olefins and thereafter sep arating the parafiins and olefins by distillation or other suitable means.

In another embodiment, the present invention is directed to and advantageously presents a method for treating a mixture of C5 olefin hydrocarbons containing branched and straight chain olens to obtain a product stream rich in trimethylethylene which may then be dehydrogenated to isoprene. A C5 hydrocarbon fraction obtained in petroleum processing and particularly' that obtained from cracking operations contain a mixture of parains, normal and iso-olens and may contain as high or higher than 30 percent of trimethylethylene.

In accordance with this invention, isoprene is prepared from a mixture of C5 hydrocarbons by the process comprising selectively hydrogenating a mixture comprising C5 oleiin hydrocarbons in the presence of finely divided solid hydrogenating catalyst under relatively mild hydrogenating conditions such that the straight chain or normal olens will be selectively hydrogenated in preference to the branched chain or isoolefins contained in the feed. Thereafter a branched chain trimethylethylene-rich stream is recovered from the selective hydrogenation step and further treated under dehydrogenating conditions to produce isoprene.

The selective hydrogenation of a mixture of C5 olefin hydrocarbons is concerned primarily with the selective hydrogenation of pentene-2 such that hydrogenation of trimethylethylene contained in the feed is restricted to a minimum. While there may be some hydrogenation of trimethylethylene taking place, this is not too disadvantageous since a valuable isomer product, isopentane, will be obtained as a result of this hydrogenation which is readily separable by simple distillation means or other suitable separating means. However, it is preferred and atent it is to be understood that in accordance with this inn vention the selective hydrogenation is to be effected under conditions to minimize hydrogenation of iso-olefins in preference to normal oletins. The hydrogenating conditions are generally mild hydrogenating conditions employing an excess of hydrogen to hydrocarbon feed which may be as high as 20:1; a temperature in the range of from about F. to about 600 F. with the preferred temperature being in the range of from about l00 F. to about 400 F.; pressures of from about 50 p.s.i.g. to about 1000 p.s.i.g., and preferably from about 100 p.s.i.g. to about 200 p.s.i.g.; and space velocities in the range of from about .005 to about 10 v./v./hr. The hydrogen to be employed in the process of this invention may be obtained from any suitable extraneous source such as a re forming process or a portion or all of the hydrogen obtained from the efiiuent ot` the dehydrogena'tion step herein described may be employed in the selective hydrogenation step with or without the addition of make-up hydrogen.

Suitable catalysts for the selective hydrogenation step of this invention are those which have activity under relatively mild hydrogenating conditions and the exact conditions will, of course depend on the particular catalyst being employed and will be milder for the more active than for the less active catalyst. It is contemplated, therefore, employing nely divided solid hydro genating catalysts either in xed or fluid beds, such as nickel, platinum, iron and cobalt containing catalysts either as oxides and/or suldes, which catalysts may be employed either alone or in admixture with one or more elements of the 6th Group of the Periodic Table. Although it is not essential, it is to be understood that the catalyst employed in the selective hydrogenation reaction may be pretreated with hydrogen prior to the hydrogenation step in order to increase its activity.

The selective hydrogenation of a mixture of branched and straight chain C5 olcns and particularly those obtained in a C5 hydrocarbon fraction in accordance with this invention produces a resultant mixture of paratiins rich in trimethylethylene which may then be readily separated by distillation or the mixture may be charged directly to a selective dehydrogenating treating step. When passing the resultant product of the selective hydragenating step directly to a dehydrogena'tion step, the operating conditions in the dehydrogenation step are selected such that selective dehydrogenation of the trimethyletnylene will be accomplished to produce a resultant product containing isoprene which may then be separated by fractional distillation or other :suitable meth-y ods. The dehydrogenation step of this invention for the conversion of trimethylethylene to isoprene contemplates the usage of a wide variety of dehydrogenation catalysts including chromia-a1urnina, supported palladium or plati num catalysts and calcium-nicited-phosphate catalysts. Operating conditions employed in the selective dchydro0 genation step include temperatures in the range of from about 700 F. to about 1250 F., preferably from about 900 F. to about 1100 F.; a pressure in the range of from about l0" mercury to about 100 p.s.i.g., preferably from about 1 p.s.i.g. to about 10 p.s.i.g.; and space veloc ities in the range of from about 0.1 v./v'./hr. to about 100 v./v./hr., preferably 1 to 10 v./v./hr.

Having thus provided a general description of the im proved method and combination of steps contemplated by this invention, reference `is now had to the following 4examples which provide a better understanding of the inven tion.

Example 1 A C5 olefin cut fractionated from a catalytic cracking operation and comprising 10.3 percent pentene-l, 40.2

Percent n-Pentane 45.9 Pentene-l and pentene-2 5.1 i-Pentane 23.1 Trimethylethylene 25.9

Separation of the effluent containing parains and olefins may be readily accomplished in view of the change in boiling point by either fractionation or suitable extraction steps to obtain an olefin fraction comprising 83.5% trimethylethylene and 16.5% pentene-l and pentene-2. The ratio of trimethylethylene to other pentenes in the product efliuent is, therefore, about :1 compared to a ratio of just below l in the original feed.

In Example 1 above, which is self-explanatory, the selective hydrogenation was effected in a single stage hydrogenation zone. It is contemplated, however, employing one or more stages of hydrogenation such as a plurality of stages either with or without separation of parains from olefins between stages.

Example 2 below presents the results obtained when employing a plurality of selective hydrogenation zones in sequence.

Example 2 Percent Pentanes 85.9 Pentene-l and pentene-2 .5 Trimethylethylene 13.

The separation of parafins from olefins in the product effluent of the second hydrogenation step may be readily separated by either distillation or other suitable methods to obtain an olefin fraction comprising 96.5 percent trimethylethylene and 3.5 percent other pentenes. Accordingly, the ratio of trimethylethylene to other pentenes becomes about 27.6 compared to just below 1 in the original feed material passed to the first hydrogenation step.

It is apparent from the above examples that applicant has provided a novel, improved and effective method for the separation of a mixture of C5 hydrocarbons containing straight and branched chain olefins by the selective hydrogenation of the straight chain hydrocarbons in preference to branched chain hydrocarbons. Although Example 2 shows passing the total efiiuent ofthe first hydrogenation step to the second hydrogenation step, it is to be understood, as hereinbefore indicated, that separation of parains from olefins may be effected between stages and prior to passing the olefin-rich stream to the next succeeding selective hydrogenation step.

The figure presented herewith is a plot of conversion of trimethylethylene as a function of conversion of pentene-2 over cobalt-molybdena hydrogenation catalysts. When no selectivity exists during hydrogenation, equal conversion, corresponding to the dotted line in the figure would prevail. As selectivity is increased in the desired direction conversion of pentene-Z will become greater than that of trimethylethylene. Points along the curves were obtained by changing conversion levels by adjustments of temperature and space velocity. Selectivity of pentene-2 hydrogenation was demonstrated and 100% pentene-Z conversion may be obtained in accordance with this invention.

Having thus given a general description of my invention and specific examples directed thereto, it is rto be understood that the invention is not to be limited thereto, but that any modifications and variations may be made thereto without departing from the scope and spirit there- I claim:

1. A method for recovering C5 iso-olefins from normal C5 olens which comprises passing a mixture of C5 hydrocarbons containing normal and iso-olefins with hydrogen in contact with a cobalt-molybdenum-oxide catalyst under conditions of a space velocity in the range of about .005 to about 10 v./v./hr., a temperature in the range of -about to about 400 F., and a pressure in the range of about 100 to about 200 p.s.i.g., said space velocity and temperature being adjusted within the aforementioned ranges to hydrogenate normal olefins in preference to C5 iso-olefins and recovering from said product of the hydrogenation step a stream richer in said C5 isoolefins than the feed to said hydrogenation step.

2. The process of claim 1 in which said mixture of C5 hydrocarbons containing normal and iso-olefins with hydrogen is passed sequentially through a plurality of hydrogenation zones containing a cobalt-molybdenum-oxide catalyst.

3. A method of recovering trimethylethylene from normal C5 olefins which comprises passing a mix-ture of C5 hydrocarbons containing trimethylethylene and normal C5 olefins with hydrogen in contact with a cobalt-molybdenum-oxide catalyst under conditions of a space velocity in the range of about 0.005 to about 10 v./v./hr., a temperature in the range of about 100 to about 400 F., and a pressure in the range of about 100 to about 200 p.s.i.g., said space velocity and temperature being adjusted within the aforementioned ranges to hydrogenate said normal C5 olefins in preference to said trimethylethylene, recovering from the product of said hydrogenation step an olefin fraction richer in trimethylethylene than said mixture of C5 hydrocarbons passed as feed to said hydrogenation step, passing said olefin fraction richer in trimethylethylene with hydrogen through at least a second hydrogenation step to hydrogenate normal olefins in preference to said trimethylethylene and thereafter recovering from the product of said hydrogenation Istep a trimethylethylene rich fraction for dehydrogenation to isoprene.

4. A method of recovering trimethylethylene from normal C5 hydrocarbons containing Z-pentenes and trimethylethylene which comprises passing such a mixture with hydrogen in contact with a cobalt-molybdenumoxide catalyst under hydrogenation conditions of a space velocity in the range of about .005 to about 10 v./v./hr., a temperature in the range of about 100 to about 400 F., and a pressure in the range of vabout 100 to about 200 p.s.i.g., said space velocity and temperature being adjusted within the aforementioned ranges to hydrogenate said 2- pentenes in preferenece to said trimethylethylene and thereafter recovering a trimethylethylene rich fraction from the product of said hydrogenation for dehydrogenation to produce isoprene. t

References Cited in the file of this patent UNITED STATES PATENTS 2,420,030 Brandon May 6, 1947 2,495,648 Voge et al. Ian. 24, 1950 2,900,429 Heinemann et al Aug. 18, 1959 OTHER REFERENCES Mamadalieb et al.: Chemical Abstracts, vol. 52 (1958), column 5279. 

1. A METHOD FOR RECOVERING C5 ISO-OLEFINS FROM NORMAL C5 OLEFINS WHICH COMPRISES PASSING A MIXTURE OF C5 HYDROCARBONS CONTAINING NORMAL AND ISO-OLEFINS WITH HYDROGEN IN CONTRACT WITH A COBALT-MOLYBDENUM-OXIDE CATALYST UNDER CONDITIONS OF A SPACE VELOCITY IN THE RANGE OF ABOUT .005 TO ABOUT 10 V./V./HR., A TEMPERATURE IN THE RANGE OF ABOUT 100 TO ABOUT 400*F., AND A PRESSURE IN THE RANGE OF ABOUT 100 TO ABOUT 200 P.S.I.G., SAID SPACE VELOCITY AND TEMPERATURE BEING ADJUSTED WITHIN THE AFOREMENTIONED RANGES TO HYDROGENATE NORMAL OLEFINS IN PREFERENCE TO C5 ISO-OLEFINS AND RECOVERING FROM SAID PRODUCT OF THE HYDROGENATION STEP A STREAM RICHER IN SAID C5 ISOOLEFINS THAN THE FEED TO SAID HYDROGENATION STEP. 